Converter

ABSTRACT

A converter is provided in which the cost and size of which are reduced by a decrease in current consumption with no need of large elements or radiation fins. The converter has a switching element (Q 1 ) being connected to a DC power source through a primary winding (P) of a transformer (T), a control circuit ( 4 ) for turning on/off the switching element (Q 1 ), a diode (D 51 ) and a capacitor (C 51 ) for rectifying/smoothing the voltage induced in the secondary winding (S) of the transformer (T) to extract DC output, and a starting circuit ( 5 ) for starting the control circuit ( 4 ). The starting circuit ( 5 ) operates as a constant current circuit (Q 2 , R 1 , R 2 , ZD 1 , D 1 ) while starting the control circuit ( 4 ), and as a constant voltage circuit (Q 2 , R 1 , R 2 , ZD 2 , D 1 ) after starting the control circuits ( 4 ).

TECHNICAL FIELD

The present invention relates to a converter, and particularly, to astart-up circuit provided for a DC-DC converter.

BACKGROUND TECHNOLOGY

FIG. 1 shows an example of a conventional DC-DC converter of that type.In FIG. 1, a sinusoidal wave voltage from an AC power source 1 isrectified and smoothed by a full-wave rectifying circuit 2 and acapacitor C1, to produce a DC voltage. The produced DC voltage is passedthrough a switching element Q1 and is applied to a primary winding P ofa transformer T. The switching element Q1 is ON/OFF-controlled accordingto a drive signal from a control circuit 4. Although not shown, thecontrol circuit 4 can control various circuits such as an output voltagefeedback circuit, an output current feedback circuit, an outputovervoltage protection circuit, an output voltage decrease detectioncircuit, and an overheat protection circuit.

The DC voltage generated by the capacitor C1 turns on a constant-currentstart-up circuit 5 consisting of a switching element Q2 made of aMOSFET, a resistor R1, a resistor R2, a Zener diode ZD1, and a diode D1.Namely, the DC voltage is passed through the resistor R1 and is appliedto a gate of the switching element Q2 to turn on the switching elementQ2, to thereby pass a constant current through a route of the switchingelement Q2, resistor R2, diode D1, and capacitor C2. This results incharging the capacitor C2. When the voltage of the capacitor C2 reachesa start-up voltage (for example, 16 V) of the control circuit 4, thecontrol circuit 4 starts to output a drive signal to the switchingelement Q1.

In response to the drive signal, the switching element Q1 starts to turnon and off. When the switching element Q1 is ON, the voltage is appliedto the primary winding P of the transformer T, which accumulates energy.

When the switching element Q1 is OFF, the energy accumulated in thetransformer T is discharged as electrical energy from a secondarywinding S of the transformer T. This voltage is rectified and smoothedby a diode D51 and a capacitor C51, to provide a required DC voltage.The transformer T has a tertiary winding C serving as a power source forthe control circuit 4. A voltage generated by the tertiary winding C isrectified and smoothed by a diode D2 and the capacitor C2, to provide asource voltage for the control circuit 4.

The start-up circuit 5 consumes large power because it receives acurrent from the high-voltage power source. If a large start-up currentis needed to shorten a start-up time of the DC-DC converter, it willinvolve increased energy. Accordingly, a start-up control circuit 6having a diode 3, a resistor R3, a resistor R4, a capacitor C3, and aswitching element Q3 detects a start of the DC-DC converter(corresponding to a start of the control circuit 4) according to avoltage generated by the tertiary winding C of the transformer T, andaccording to the detected voltage, turns on the switching element Q3.This brings a gate bias voltage of the switching element Q2 nearly to aground voltage, to turn off the switching element Q2. This results inturning off the start-up circuit 5. In this way, the start-up circuit 5is turned off after a start of the DC-DC converter, to reduceunnecessary power consumption.

DISCLOSURE OF INVENTION

The conventional DC-DC converter, however, has unsolved problemsmentioned below. For example, if the DC-DC converter develops anoverheat or overvoltage state and the control circuit 4 is latched(storing data in a flip-flop FF41) to stop operation, the controlcircuit 4 provides no drive signal to the switching element Q1. Then,the tertiary winding C of the transformer T generates no voltage, andtherefore, the switching element Q3 in the start-up control circuit 6 isnot turned on. In this case, the start-up circuit 5 continuouslyoperates to cause a large energy loss. For this, even a normal operationthat may involve no loss needs large elements and large radiation finsto cope with an overheat or overvoltage state. This increases the costand size of the converter.

If the DC-DC converter develops an overload state, a protection circuit10 decreases the voltage of the tertiary winding C of the transformer T.If this voltage reaches a stop voltage to stop the control circuit 4,the control circuit 4 stops to stop the DC-DC converter. If the DC-DCconverter stops, the switching element Q3 in the start-up controlcircuit 6 turns off to turn on the start-up circuit 5. While the DC-DCconverter keeps the overload state, these operations are repeated sothat the DC-DC converter repeatedly starts and stops. A repetitionperiod of the start and stop is determined by a constant current valueof the start-up circuit 5, a capacity of the capacitor C2, and the like.If the repetition period is short, the start-up circuit 5 must bearlarge load to increase a loss. To elongate the repetition period, theconstant current value of the start-up circuit 5 and the capacity of thecapacitor C2 must be increased. This, however, elongates the start-uptime.

According to the conventional DC-DC converter of this type, the DC-DCconverter operates for a while after the AC power source 1 is turned offonly with energy accumulated in the capacitor C1. If the voltage of thecapacitor C1 drops so that the DC-DC converter is unable to maintain anoutput, the voltage of the tertiary winding C of the transformer Tdecreases. When this voltage reaches the stop voltage of the controlcircuit 4, the control circuit 4 stops to stop the DC-DC converter. Ifthe DC-DC converter stops, the switching element Q3 in the start-upcontrol circuit 6 turns off to again turn on the start-up circuit 5. Ifthe voltage of the capacitor C2 reaches a start-up voltage of thecontrol circuit 4, the control circuit 4 starts to again start the DC-DCconverter. Then, the switching element Q3 in the start-up controlcircuit 6 again turns on to turn off the start-up circuit 5. The voltageof the capacitor C1, however, is low, and therefore, the voltage of thetertiary winding C of the transformer T is too low to maintain theoperation of the control circuit 4. As a result, the control circuit 4again stops. In this way, after the AC power source 1 is turned off, theDC-DC converter repeats the start and stop operations for a while.

The present invention provides a converter capable of reducing currentconsumption even if a DC-DC converter develops an overheat orovervoltage state, employing no large elements or large radiation fins,and reducing the cost and size thereof.

The present invention also provides a converter capable of surelystopping a DC-DC converter when an AC power source is turned off.

To achieve the objects, an invention of claim 1 provides a converterhaving a first switching element connected to a DC power source througha primary winding of a transformer, a control circuit to conduct ON/OFFcontrol on the first switching element, an output rectifying/smoothingcircuit to rectify and smooth a voltage induced on a secondary windingof the transformer and provide a. DC output, and a start-up circuit tostart the control circuit. The start-up circuit operates as a constantcurrent circuit when starting the control circuit and as a constantvoltage circuit after starting the control circuit.

An invention of claim 2 provides the converter with a start-up controlcircuit to detect a start of the control circuit according to a voltagegenerated by a tertiary winding of the transformer and switch theconstant current circuit operation to the constant voltage circuitoperation, and a voltage supply part to supply the voltage generated bythe tertiary winding of the transformer to the control circuit.

According to an invention of claim 3, the control circuit, if broughtinto a latched state by a protection circuit, provides the start-upcontrol circuit with a latch signal whose voltage is lower than thestart-up voltage and corresponds to a voltage to maintain the latchedstate. The start-up control circuit operates in response to the latchsignal, to make the start-up circuit operate as the constant voltagecircuit.

An invention of claim 4 provides the converter with arectifying/smoothing circuit connected to an AC power source, to rectifyand smooth AC power and produce DC power. The AC power source andrectifying/smoothing circuit form the DC power source.

An invention of claim 5 provides the converter with a rectifying circuitconnected to an AC power source, to full-wave-rectify an AC voltage anda power-factor improving circuit to receive a full-wave-rectified outputfrom the rectifying circuit through a choke coil, turn on and off,rectify, and smooth the received output with a second switching element,and provide a DC output. The AC power source, rectifying circuit, andpower-factor improving circuit form the DC power source.

According to an invention of claim 6, a bias voltage or current to acontrol terminal of a start-up switching element in the start-up circuitis supplied from the DC power source.

According to an invention of claim 7, a bias voltage or current to acontrol terminal of a start-up switching element in the start-up circuitis supplied from the AC power source.

According to an invention of claim 8, a bias voltage or current to acontrol terminal of a start-up switching element in the start-up circuitis supplied from a front stage of the power-factor improving circuit.

An invention of claim 9 provides a converter having arectifying/smoothing circuit connected to an AC power source, to rectifyand smooth an AC voltage, a first switching element connected to anoutput side of the rectifying/smoothing circuit through a primarywinding of a transformer, a control circuit to conduct ON/OFF control onthe first switching element, a rectifying/smoothing circuit to rectifyand smooth a voltage induced on a secondary winding of the transformerand provide a DC output, and a start-up circuit to start the controlcircuit. A bias voltage or current to a control terminal of a start-upswitching element in the start-up circuit is supplied from the AC powersource.

An invention of claim 10 provides a converter having a rectifyingcircuit connected to an AC power source, to full-wave-rectify an ACvoltage, a power-factor improving circuit to receive afull-wave-rectified waveform from the rectifying circuit through a chokecoil, turn on and off, rectify, and smooth the received output with asecond switching element, and provide a DC output, a first switchingelement connected to an output side of the power-factor improvingcircuit through a primary winding of a transformer, a control circuit toconduct ON/OFF control on the first switching element, an outputrectifying/smoothing circuit to rectify and smooth a voltage induced ona secondary winding of the transformer and provide a DC output, and astart-up circuit to start the control circuit. A bias voltage or currentto a control terminal of a start-up switching element in the start-upcircuit is supplied from a front stage of the power-factor improvingcircuit.

An invention of claim 11 provides the start-up circuit with a capacitorto hold the bias voltage or current for a half period of a frequency ofthe AC power source.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing an arrangement of a conventional DC-DCconverter;

FIG. 2 is a view showing an arrangement of a DC-DC converter accordingto a first embodiment of the present invention;

FIG. 3 is a timing chart showing parts of the DC-DC converter accordingto the first embodiment of the present invention;

FIG. 4 is a view showing an arrangement of a converter composed of apower-factor improving converter and a DC-DC converter according to asecond embodiment of the present invention;

FIG. 5 is a view showing an arrangement of a DC-DC converter accordingto a third embodiment of the present invention;

FIG. 6 is a timing chart showing parts of the DC-DC converter accordingto the third embodiment of the present invention;

FIG. 7 is a view showing an arrangement of a converter composed of apower-factor improving converter and a DC-DC converter according to afourth embodiment of the present invention;

FIG. 8 is a view showing an arrangement of a converter composed of apower-factor improving converter and a DC-DC converter according to afifth embodiment of the present invention;

FIG. 9 is a view showing an arrangement of a DC-DC converter accordingto a sixth embodiment of the present invention;

FIG. 10 is a view showing an arrangement of a converter composed of apower-factor improving converter and a DC-DC converter according to aseventh embodiment of the present invention;

FIG. 11 is a view showing an arrangement of an example of a power-factorimproving converter; and

FIG. 12 is a timing chart explaining operation of the power-factorimproving converter.

BEST MODE OF IMPLEMENTATION

Converters according to embodiments of the present invention will beexplained with reference to the drawings.

First Embodiment

FIG. 2 is a view showing an arrangement of a DC-DC converter accordingto the first embodiment of the present invention. The DC-DC converter ofFIG. 2 is different from the conventional DC-DC converter of FIG. 1 inthat it additionally has a Zener diode ZD2 and a diode D4. In the DC-DCconverter of FIG. 2, the same parts as those of the conventionalconverter of FIG. 1 are represented with the same reference marks.

In FIG. 2, a sinusoidal wave voltage from an AC power source 1 isrectified and smoothed by a full-wave rectifying circuit 2 and acapacitor C1, to provide a DC voltage. The DC voltage is supplied to theDC-DC converter 3 a, which converts the input DC voltage into another DCvoltage that is output from output terminals +Vout and −Vout.

The arrangement of the DC-DC converter 3 a will be explained in detail.The capacitor C1 is connected through a primary winding P of atransformer T to a switching element Q1 made of a MOSFET. The switchingelement Q1 is turned on and off under PWM control by a control circuit4. A secondary winding S of the transformer T is connected to arectifying/smoothing circuit made of a diode D51 and a capacitor C51.The rectifying/smoothing circuit rectifies and smoothes a voltage(ON/OFF-controlled pulse voltage) induced on the secondary winding S ofthe transformer T and provides a DC output from the output terminals+Vout and −Vout. A protection circuit 10 detects an overvoltage state oran overheat state of the DC-DC converter 3 a according to the DC outputof the rectifying/smoothing circuit and puts the control circuit 4 intoa latched state to stop the same.

A first end of the capacitor C1 is connected to a start-up circuit 5 aconsisting of a switching element Q2 made of a MOSFET, a resistor R1, aresistor R2, a Zener diode ZD1, the Zener diode ZD2, and a diode D1. Adrain of the switching element Q2 is connected to the first end of thecapacitor C1, and between the drain and gate (a control terminal of thepresent invention) of the switching element Q2, the resistor R1 isconnected. A source of the switching element Q2 is connected to a firstend of the resistor R2. A second end of the resistor R2 is connected toan anode of the Zener diode ZD1 and an anode of the diode D1. A cathodeof the diode D1 is connected to a first end of a capacitor C2 thatsupplies a voltage to start the control circuit 4. A cathode of theZener diode ZD1 is connected to the gate of the switching element Q2 anda cathode of the Zener diode ZD2. An anode of the Zener diode ZD2 isconnected to a collector of a switching element Q3 made of a bipolartransistor in a start-up control circuit 6.

The switching element Q2, resistor R1, resistor R2, Zener diode ZD1, anddiode D1 operate as a constant current circuit. The switching elementQ2, resistor R1, resistor R2, Zener diode ZD2, and diode Dl operate as aconstant voltage circuit. With the Zener diode ZD2, the start-up circuit5 a can operate as the constant current circuit when starting thecontrol circuit 4 and as the constant voltage circuit after starting thecontrol circuit 4. A breakdown voltage of the Zener diode ZD2 isadjusted to a constant voltage (for example, 8 V) that is smaller than avoltage generated by a tertiary winding C of the transformer T.

The control circuit 4 starts to operate according to a voltage (forexample, 16 V) supplied from the capacitor C2 and provides the switchingelement Q1 with a drive signal from an output terminal 4 c. If the DC-DCconverter 3 a develops an overheat state, an overvoltage state, or anoverload state, an input terminal 4 d of the control circuit 4 receivesa protection signal from the protection circuit 10, and a flip-flop FF41establishes a latched state (or holding data at certain timing). As aresult, the control circuit 4 stops, and no drive signal is providedfrom the output terminal 4 c to the switching element Q1. At the sametime, the control circuit 4 provides a latch signal through the diode D4to a first end of a capacitor C3 in the start-up control circuit 6. Whenthe control circuit 4 is in the latched state, minimum power (forexample, a power source voltage of 6 V) to keep the latched state issupplied from the constant voltage circuit of the start-up circuit 5 ato power source input terminals 4 a and 4 b of the control circuit 4through the capacitor C2.

A first end of the tertiary winding C of the transformer T is connectedto an anode of a diode D2, and a cathode of the diode D2 is connected tothe first end of the capacitor C2 and the control circuit 4. Thestart-up control circuit 6 employs a diode D3, a resistor R3, a resistorR4, and the capacitor C3 to rectify and smooth a voltage from thetertiary winding C and apply the voltage to a base of the switchingelement Q3. Namely, when detecting a start of the DC-DC converter 3 a,the start-up control circuit 6 turns on the switching element Q3 tobreak down the Zener diode ZD2 and operate the start-up circuit 5 a asthe constant voltage circuit. If the DC-DC converter 3 a shows anoverheat state, overvoltage state, or overload state, the first end ofthe capacitor C3 of the start-up control circuit 6 receives a latchsignal from the control circuit 4 through the diode D4. According to avoltage based on the latch signal, the switching element Q3 is turned onto break down the Zener diode ZD2 to operate the start-up circuit 5 a asthe constant voltage circuit.

Operation of the DC-DC converter according to the first embodiment withthe above-mentioned arrangement will be explained with reference to thetiming chart of FIG. 3. In FIG. 3, AC indicates a sinusoidal wavevoltage of the AC power source 1, C2 a voltage of the capacitor C2, V0 avoltage of the capacitor C51, C1 a voltage of the capacitor C1, and C3 avoltage of the capacitor C3.

At time to, the sinusoidal wave voltage from the AC power source 1 isapplied to the full-wave rectifying circuit 2. The sinusoidal wavevoltage is full-wave rectified by the full-wave rectifying circuit 2 andis smoothed by the capacitor C1. As a result, the DC voltage of thecapacitor C1 steeply increases to a given DC voltage. This DC voltage isapplied through the switching element Q1 to the primary winding P of thetransformer T.

On the other hand, the DC voltage generated at the ends of the capacitorC1 operates the constant current circuit consisting of the switchingelement Q2, resistor R1, resistor R2, Zener diode ZD1, and diode D1.Namely, the DC voltage is applied through the resistor R1 to the gate ofthe switching element Q2 to turn on the switching element Q2 and pass aconstant current through a route of the switching element Q2, resistorR2, diode D1, and capacitor C2. The capacitor C2 is charged, and thevoltage of the capacitor C2 linearly increases to reach a start-upvoltage Vthon (for example, 16 V) to start the control circuit 4 at timet1. Then, the control circuit 4 starts to provide the switching elementQ1 with a drive signal. The charge of the capacitor C2 is consumed tooperate the control circuit 4, and therefore, the voltage of thecapacitor C2 gradually drops to a constant voltage V1 (for example, 12V) at time t2.

On the other hand, at time t1, the switching element Q1 starts to turnon and off in response to the drive signal. When the switching elementQ1 is ON, a voltage is applied to the primary winding P of thetransformer T, which accumulates energy. When the switching element Q1is turned off, the energy accumulated in the transformer T is dischargedas electrical energy from the secondary winding S of the transformer T.This voltage is rectified and smoothed by the diode D51 and capacitorC51, to provide a required DC voltage. As a result, the voltage V0 ofthe capacitor C51 increases from time t1 to time t2 and becomes aconstant value at time t2.

A voltage generated by the tertiary winding C is rectified and smoothedby the diode D2 and capacitor C2, and an obtained voltage of, forexample, 12 V is applied to the control circuit 4. The voltage of thecapacitor C3, which is obtained by rectifying and smoothing the voltagegenerated by the tertiary winding C, also increases like the voltage V0of the capacitor C51 and reaches a constant value at time t2. Thevoltage of the capacitor C3 is applied to the base of the switchingelement Q3.

Namely, detecting a start of the DC-DC converter 3 a turns on theswitching element Q3, and therefore, the Zener diode ZD2 breaks down tooperate the start-up circuit 5 a as the constant voltage circuit. Atthis time, a cathode voltage of the Zener diode ZD2 is about 8 V. Inconsideration of a voltage drop of the Zener diode ZD1, the anode of thediode D1 receives about 6 V. When the control circuit 4 is operating, avoltage of 16 V is being applied thereto, and therefore, the cathode ofthe diode D1 receives about 16 V. As a result, the diode D1 is put in areversely biased state to pass no current. Namely, after the start ofthe control circuit 4, the diode D1 is turned off to cause no loss.

At time t3, the AC power source 1 is turned off, and the voltage of thecapacitor C1 gradually drops from time t3. The voltage of the capacitorC2 maintains a constant voltage V1 until just before time t4 and dropsto a stop voltage Vthoff (for example, 10 V) at time t4 to stop thecontrol circuit 4. Then, the control circuit 4 stops. The voltage of thecapacitor C51 and the voltage of the capacitor C3 are constant up totime t4, and after time t4, decrease because the control circuit 4 isstopped.

After time t4, the voltage of the capacitor C3 drops below a thresholdvoltage Vth and the transistor Q3 is turned off so that the Zener diodeZD2 does not break down (OFF state). Namely, the start-up circuit 5 a isswitched from the constant voltage circuit to the constant currentcircuit, to pass a current through the diode D1. As a result, thevoltage of the capacitor C2 linearly increases to reach the start-upvoltage Vthon at time t5. Then, the control circuit 4 is started toprovide the switching element Q1 with a drive signal. At this time, theAC power source 1 is OFF, and therefore, the voltage of the capacitor C1decreases and the voltage of the capacitor C2 gradually drops below thestop voltage Vthoff.

On the other hand, at time t5, the switching element Q1 starts to turnon and off in response to the drive signal, and the voltage of thecapacitor C51 and the voltage of the capacitor C3 increase. However,they decrease as the voltage of the capacitor C1 decreases because theAC power source 1 is OFF.

At time t6, the AC power source 1 is turned on. Operations of the partsbetween time t6 and time t9 are the same as those between time t0 andtime t3, and therefore, the detail explanation thereof is omitted.

At time t10, if the DC-DC converter 3 a develops an overvoltage statedue to some reason, the voltage of the capacitor C2 becomes higher (OV1)than the start-up voltage Vthon. The voltage of the capacitor C51 andthe voltage of the capacitor C3 also become higher than theabove-mentioned constant voltages.

At this time, the protection circuit 10 detects the voltage of thecapacitor C51, determines that the voltage is abnormal, puts the controlcircuit 4 into a latched state; and stops the operation thereof. Then,an output terminal 4 e of the control circuit 4 outputs at time t11 alatch signal of a voltage (for example, 6 V) higher than the thresholdvoltage Vth. The latch signal is applied through the diode D4 to thecapacitor C3 in the start-up control circuit 6. Due to this, theswitching element Q3 maintains an ON state even if the control circuit 4is in the latched state. Since the switching element Q3 keeps the ONstate, the start-up circuit 5 a operates as the constant voltagecircuit.

At this time, the voltage of the cathode of the Zener diode ZD2 is about8 V, and therefore, the diode D1 is turned on. A voltage of about 6 V isapplied to the cathode of the diode D1 and the capacitor C2, and thisvoltage is applied to the power source input terminals 4 a and 4 b ofthe control circuit 4. Accordingly, the voltage of the capacitor C2gradually drops below the stop voltage Vthoff and reaches about 6 V attime t12. Thereafter, it maintains a constant value. Namely, when thecontrol circuit 4 is in a latched state, minimum power (a power sourcevoltage of 6 V in this example) to maintain the latched state must besupplied to the control circuit 4. This minimum power is supplied fromthe start-up circuit 5 a. At this time, the start-up circuit 5 a isoperating as the constant voltage circuit. This is advantageous inefficiency because no large current flows. In FIG. 3, VZD2 is a voltageby the Zener diode ZD2, and VD4 is a voltage by the diode D4.

As mentioned above, the DC-DC converter 3 a according to the firstembodiment employs the Zener diode ZD2, so that the start-up circuit 5 aoperates as the constant current circuit at a start of the controlcircuit 4 and as the constant voltage circuit after the start of thecontrol circuit 4. The breakdown voltage of the Zener diode ZD2 isadjusted to a constant voltage smaller than a voltage generated by thetertiary winding C of the transformer T. Accordingly, the diode D1 isOFF during a steady state, to cause no loss. As a result, the converterneeds no large elements or large radiation fins, and therefore, canreduce the cost and size thereof.

In a latched state, the control circuit 4 provides the start-up controlcircuit 6 with a latch signal through the diode D4, so that theswitching element Q3 can maintain an ON state and the start-up circuit 5a can operate as the constant voltage circuit even if the controlcircuit 4 is in the latched state. At this time, the start-up circuit 5a passes no large current and can supply minimum power to the controlcircuit 4.

According to the conventional DC-DC converter of FIG. 1, the DC-DCconverter is stopped if it develops an overload state. Then, theswitching element Q3 in the start-up control circuit 6 is turned off toagain turn on the start-up circuit 5. In this way, the DC-DC converterrepeats start and stop. According to the first embodiment, if the DC-DCconverter 3 a develops an overload state and stops (the control circuit4 also stops to establish a latched state), the control circuit 4outputs a latch signal to turn on the switching element Q3 in thestart-up control circuit 6 so that the start-up circuit 5 a operates asthe constant voltage circuit. This stops restarting and prevents theDC-DC converter from repeating start and stop.

Second Embodiment

FIG. 4 is a view showing an arrangement of a converter according to thesecond embodiment of the present invention consisting of a power-factorimproving converter and a DC-DC converter. In the converter of thesecond embodiment, the power-factor converter 7 has a choke coil L1, aswitching element Q4, a diode D7, a power-factor improving controlcircuit (PFC control circuit) 71, and a capacitor C1 and is arranged infront of the DC-DC converter 3 a.

The DC-DC converter 3 a has already been explained in the firstembodiment of FIG. 2, and therefore, will not be explained here. Onlythe power-factor improving converter 7 will be explained. The details ofthe arrangement of the power-factor improving converter 7 will beexplained later.

In FIG. 4, the choke coil L1, switching element Q4, diode D7,power-factor improving control circuit (PFC control circuit) 71, andcapacitor C1 form a step-up chopper circuit. The step-up chopper circuitsteps up an input voltage from a full-wave rectifying circuit 2 andprovides, from the capacitor C1, a constant DC voltage.

A first end of the full-wave rectifying circuit 2 is connected to afirst end of the choke coil L1. A second end of the choke coil L1 isconnected to an anode of the diode D7. A cathode of the diode D7 isconnected to a first end of the capacitor C1. A connection point betweenthe second end of the choke coil L1 and the anode of the diode D7 isconnected to a drain of the switching element Q4 made of a MOSFET. Asource of the switching element Q4 is grounded and a gate thereof isconnected to the PFC control circuit 71. The PFC control circuit 71receives source power from the DC-DC converter 3 a and conducts ON/OFFcontrol on the switching element Q4, to control an input currentwaveform to a sinusoidal wave that follows an input voltage waveform andstep up an input voltage to a constant DC voltage, which is supplied tothe capacitor C1. The DC-DC converter 3 a receives a DC voltage from thecapacitor C1 and operates like that of the first embodiment of FIG. 2.

In this way, the converter according to the second embodiment with thepower-factor improving converter 7 steps up an input voltage to aconstant DC voltage and shapes a current from an AC power source 1 intoa sinusoidal wave current waveform that follows a voltage of the ACpower source 1, to thereby greatly improve a power factor.

Third Embodiment

FIG. 5 is a view showing a DC-DC converter according to the thirdembodiment of the present invention. In the DC-DC converter of the firstembodiment, a bias voltage to the gate of the switching element Q2 ofthe start-up circuit 5 a is supplied from the capacitor C1. Unlike this,in the DC-DC converter of the third embodiment, a bias voltage to a gateof a switching element Q2 of a start-up circuit 5 b is supplied from anAC power source 1 through diodes D5 and D6 and a resistor R1.

In the start-up circuit 5 b, a first end of the AC power source 1 isconnected to an anode of the diode D5. A cathode of the diode D5 isconnected through the resistor R1 to the gate of the switching elementQ2. A second end of the AC power source 1 is connected to an anode ofthe diode D6. A cathode of the diode D6 is connected through theresistor R1 to the gate of the switching element Q2. The other parts ofthe DC-DC converter of FIG. 5 are the same as those of the DC-DCconverter of FIG. 2, and therefore, the same parts are represented withthe same reference marks to omit their explanations.

According to the DC-DC converter of the third embodiment, asinusoidal-wave voltage from the AC power source 1 is rectified by thediodes D5 and D6, and the rectified voltage is passed through theresistor R1 and is applied as a bias voltage to the gate of theswitching element Q2 in the start-up circuit 5 b. Accordingly, if the ACpower source 1 is turned off, the bias voltage to the gate of theswitching element Q2 is instantaneously stopped. As a result, thestart-up circuit 5 b never turns on again, and the DC-DC converter neverrepeatedly starts and stops.

FIG. 6 is a timing chart showing the parts of the DC-DC converteraccording to the third embodiment of the present invention. According tothe third embodiment, the start-up circuit 5 b does not start again, andtherefore, the voltage of a capacitor C2 around time t5 graduallydecreases without increasing. The voltage of a capacitor C51 and thevoltage of a capacitor C3 also gradually decrease without increasing.

According to the third embodiment, the two diodes D5 and D6 areconnected to the ends of the AC power source, respectively. For example,only one of the diodes D5 and D6 may be employed to simplify thestructure. Alternatively, none of the two diodes D5 and D6 may beemployed, so that the AC power source 1 is directly connected to theresistor R1.

Fourth Embodiment

FIG. 7 is a view showing an arrangement of a converter according to thefourth embodiment of the present invention, consisting of a power-factorimproving converter and a DC-DC converter. The converter of the fourthembodiment includes the power-factor improving converter 7 and DC-DCconverter 3 b. Namely, the converter of the fourth embodiment adds thepower-factor improving converter 7 of the second embodiment of FIG. 4 tothe DC-DC converter 3 b of the third embodiment of FIG. 5.

Accordingly, the converter of the fourth embodiment provides the effectof the DC-DC converter 3 b of the third embodiment and the effect of thepower-factor improving converter of the second embodiment.

Fifth Embodiment

FIG. 8 is a view showing a converter according to the fifth embodimentof the present invention, consisting of a power-factor improvingconverter and a DC-DC converter. The converter of the fifth embodimentincludes the power-factor improving converter 7 and DC-DC converter 3 c.In a start-up circuit 5 c of the DC-DC converter 3 c, a first end of afull-wave rectifying circuit 2 is connected through a resistor R1 to agate of a switching element Q2. Between the gate of the switchingelement Q2 and a second end of a resistor R2, a capacitor C4 isconnected. The capacitor C4 holds a bias voltage or current to the gateof the switching element Q2 for a half period of a frequency of an ACpower source 1.

According to the fourth embodiment of FIG. 7, a bias voltage to the gateof the switching element Q2 of the start-up circuit 5 b is obtained fromthe AC power source 1 through the diodes D5 and D6. According to thefifth embodiment, the bias voltage to the gate of the switching elementQ2 of the start-up circuit 5 c is obtained from an input stage of thepower-factor improving converter 7. In this case, the voltage from theinput stage of the power-factor improving converter 7 is a pulsatingcurrent waveform obtained by rectifying a sinusoidal-wave voltage of theAC power source 1, and therefore, can provide the same effect as thefourth embodiment.

Between the gate of the switching element Q2 and the second end of theresistor R2, the capacitor C4 is arranged. Even if a pulsating voltagefrom the full-wave rectifying circuit 2 is applied to the gate of theswitching element Q2, charge accumulated in the capacitor C4 cancompensate a bias voltage around a zero sinusoidal-wave voltage, so thatthe start-up circuit 5 c can continuously operate.

Sixth Embodiment

FIG. 9 is a view showing an arrangement of a DC-DC converter accordingto the sixth embodiment of the present invention. The sixth embodimentis characterized in that a bias voltage to a gate of a switching elementQ2 of a start-up circuit 5 d is obtained from an AC power source 1through diodes D5 and D6 and a resistor R1.

The other parts of FIG. 9 are the same as those of the DC-DC converterof FIG. 1, and therefore, the same parts are represented with the samereference marks.

According to the DC-DC converter of FIG. 9, a bias voltage to the gateof the switching element Q2 of the start-up circuit 5 d is obtained fromthe AC power source 1 through the diodes D5 and D6. If the AC powersource 1 is turned off, the start-up circuit 5 d will never start againand the DC-DC converter will never repeatedly start and stop.

Seventh Embodiment

FIG. 10 is a view showing an arrangement of a converter according to theseventh embodiment of the present invention, consisting of apower-factor improving converter and a DC-DC converter. The converter ofthe seventh embodiment includes the power-factor improving converter 7and DC-DC converter 3 e. The seventh embodiment obtains a bias voltageto a gate of a switching element Q2 of a start-up circuit 5 e from aninput stage of the power-factor improving converter 7. The other partsof FIG. 10 are the same as those of the DC-DC converter of FIG. 1, andtherefore, the same parts are depicted by the same reference marks.

A voltage from the input stage of the power-factor improving converter 7is a pulsating current waveform obtained by rectifying a sinusoidal-wavevoltage of an AC power source 1. Accordingly, if the AC power source 1is turned off, the start-up circuit 5 e will never start again and theDC-DC converter will never repeatedly start and stop.

The present invention is not limited to the above-mentioned first toseventh embodiments. The fifth embodiment inserts the capacitor C4between the gate of the switching element Q2 and the second end of theresistor R2. Insertion of the capacitor C4 is applicable to the third,fourth, sixth, and seventh embodiments.

Example of Power-Factor Improving Converter

FIG. 11 is a view showing an example of an arrangement of thepower-factor improving converter. The power-factor improving converter 7steps up an input voltage from a full-wave rectifying circuit 2 to aconstant DC voltage and outputs the constant DC voltage from a capacitorC1. It maintains the constant output voltage and controls an inputcurrent waveform to a sinusoidal wave that follows an input voltagewaveform.

In FIG. 11, a sinusoidal-wave voltage from an AC power source 1 isfull-wave rectified by the full-wave rectifying circuit 2, and thefull-wave rectified waveform is supplied to the power-factor improvingconverter 7. The DC output of the power-factor improving converter isinput to a DC-DC converter 3 a. The DC-DC converter 3 a converts the DCvoltage from the power-factor improving converter 7 into another DCvoltage and outputs the same from output terminals +Vout and −Vout.

The arrangement of the power-factor improving converter 7 will beexplained in detail. The power-factor improving converter 7 basicallyhas a step-up chopper circuit that consists of a primary winding 61 a ofa choke coil L1, a switching element Q4, a diode D7, and an outputcapacitor 65.

The choke coil L1 has the primary winding 61 a and a criticalitydetection winding 61 b. A first end of the primary winding 61 a isconnected to a first end of the full-wave rectifying circuit 2 and aresistor 51. A second end of the primary winding 61 a is connected to adrain of the switching element Q4 made of a MOSFET and an anode of thediode D7. A first end of the criticality detection winding 61 b isconnected through a resistor 60 to a positive input terminal (+) of acomparator 54. A second end of the criticality detection winding 61 b isgrounded. A cathode of the diode D7 is connected to a first end of theoutput capacitor 65 and an input terminal of the DC-DC converter 3 a.

Next, an arrangement of a PFC control circuit serving as a controlsystem of the power-factor improving converter 7 will be explained. Thepositive input terminal (+) of the comparator 54 is grounded through theresistor 60 and criticality detection winding 61 b. A negative inputterminal (−) of the comparator 54 receives a first reference voltage 53.The comparator 54 compares the input voltages with each other, and ifthe voltage of the criticality detection winding 61 b to the positiveinput terminal is lower than the first reference voltage 53, outputs alow-level set signal to a set terminal of a flip-flop 59.

The set terminal of the flip-flop 59 is connected to an output terminalof the comparator 54, a reset terminal thereof to an output terminal ofa comparator 56, and a Q-output terminal thereof to the gate terminal ofthe switching element Q4. When receiving the low-level set signal fromthe comparator 54, the flip-flop 59 outputs a high-level drive signalfrom the Q-output terminal. When receiving a high-level reset signalfrom the comparator 56, the Q-output terminal outputs a low-levelsignal.

A negative input terminal (−) of an operational amplifier 57 receives aterminal voltage of the capacitor 65 divided by resistors 66 and 67, anda positive input terminal (+) thereof receives a second referencevoltage 58. The operational amplifier 57 amplifies a difference signalbetween the divided voltage corresponding to the output voltage of thecapacitor 65 and the second reference voltage 58 and provides an errorsignal to a multiplier 55.

A first input terminal of the multiplier 55 receives afull-wave-rectified waveform from the full-wave rectifying circuit 2divided by resistors 51 and 52, and a second input terminal thereofreceives the error signal from the operational amplifier 57. Themultiplier 55 multiplies the full-wave-rectified waveform by the errorsignal and supplies a current target value Vm interlocked with thefull-wave-rectified waveform to a negative input terminal of thecomparator 56.

The negative input terminal (−) of the comparator 56 receives thecurrent target value Vm for a switching current from the multiplier 55,and a positive input terminal (+) of the comparator 56 is connected to acurrent detection resistor 63 to receive, as a current detection value,a voltage corresponding to a drain-source current of the switchingelement Q4 in an ON period. When a switching current reaches the currenttarget value Vm interlocked with the full-wave-rectified waveform, thecomparator 56 provides a high-level reset signal to the flip-flop 59.

Operation of the power-factor improving converter will be explained.When the AC power source 1 is activated, a sinusoidal-wave voltage fromthe AC power source 1 is full-wave rectified by the full-wave rectifyingcircuit 2, and the full-wave-rectified waveform is supplied to thepower-factor improving converter 7.

(1) Start-Up Operation

The positive input terminal of the comparator 54 is grounded through theresistor 60 and criticality detection winding 61 b. The negative inputterminal of the comparator 54 receives the first reference voltage 53.The comparator 54 compares the input voltages with each other. Since thevoltage to the positive input terminal is lower than the other, thecomparator 54 outputs a low-level set signal to the flip-flop 59.

In response to the set signal from the comparator 54, the flip-flop 59is set, and at timing t1 of FIG. 12, outputs a high-level drive signalfrom the Q-output terminal, to thereby turn on the switching element Q4.

At the timing t1 of FIG. 12, the switching element Q4 is turned on, anda drain voltage Vd of the switching element Q4 drops nearly to 0 V. Aswitching current flows from the full-wave rectifying circuit 2 to theground GND through the drain-source of the switching element Q4 and thecurrent detection resistor 63. As a result, the choke coil L1accumulates energy.

At this time, the switching current flowing to the switching element Q4is converted as shown in FIG. 12 into a voltage Vs by the currentdetection resistor 63 arranged between the source of the switchingelement Q4 and the ground GND. The voltage Vs is input to the positiveinput terminal of the comparator 56, which compares it with the currenttarget value Vm that is interlinked with the full-wave-rectifiedwaveform and is provided by the multiplier 55.

(2) Current Target Value Vm

An output voltage from the output capacitor 65 is divided by theresistors 66 and 67 and is input to the negative input terminal of theoperational amplifier 57. The operational amplifier 57 generates adifference signal between the divided voltage and the second referencevoltage 58, amplifies the difference signal into an error signal, andsupplies the error signal to the multiplier 55.

The full-wave-rectified waveform from the full-wave rectifying circuit 2is divided by the resistors 51 and 52 and is input to the multiplier 55.The multiplier 55 multiplies the error signal from the operationalamplifier 57 by the full-wave-rectified waveform from the full-waverectifying circuit 2, to provide a voltage serving as the current targetvalue Vm interlocked with the full-wave-rectified waveform. The currenttarget value Vm is supplied to the negative input terminal of thecomparator 56.

(3) OFF Control of Switching Element

At timing t2 of FIG. 12, a switching current detected value reaches thecurrent target value Vm interlocked with the full-wave-rectifiedwaveform. Then, the comparator 56 provides a high-level reset signal tothe flip-flop 59. In response to the reset signal from the comparator56, the flip-flop 59 is reset, and the high-level drive signal outputfrom the Q-output terminal is changed to a low-level signal to turn offthe switching element Q4.

When the switching element Q4 is turned off, energy accumulated in thechoke coil L1 and a voltage supplied from the full-wave rectifyingcircuit 2 are combined to charge the output capacitor 65 through thediode D7.

Namely, the output capacitor 65 receives a voltage stepped up higherthan a peak value of the full-wave-rectified waveform supplied by thefull-wave rectifying circuit 2.

(4) ON Control of Switching Element

When the energy accumulated in the choke coil L1 is discharged, thecriticality detection winding 61 b generates a ringing voltage to invertthe voltage of the criticality detection winding 61 b. This voltage iscompared with the first reference voltage 53 by the comparator 54. Attiming t3, the comparator 54 provides a low-level set signal to theflip-flop 59.

In response to the set signal from the comparator 54, the flip-flop 59is set to provide, at the timing t3 of FIG. 12, a drive signal to turnon the switching element Q4.

As explained above, the present invention operates the start-up circuitas a constant current circuit at a start that requires a large currentto shorten a start-up time, and at any time other than the start,operates the start-up circuit as a constant voltage circuit to reduce aload current and current consumption. The present invention needs nolarge elements nor large radiation fins, and therefore, the presentinvention can reduce the cost and size of the converter. When a biasvoltage to the start-up circuit is from an AC power source or from arectified pulsating current, the bias voltage disappears as soon as theAC power source is turned off, to surely stop the DC-DC converter.

1. A converter comprising: a first switching element being connected toa DC power source through a primary winding of a transformer and acontrol circuit to conduct ON/OFF control on the first switchingelement; and an output rectifying/smoothing circuit to rectify andsmooth a voltage induced on a secondary winding of the transformer andprovide a DC output and a start-up circuit to start the control circuit,the start-up circuit operating as a constant current circuit whenstarting the control circuit and as a constant voltage circuit afterstarting the control circuit.
 2. The converter of claim 1, comprising: astart-up control circuit to detect a start of the control circuitaccording to a voltage generated by a tertiary winding of thetransformer and switch the constant current circuit operation to theconstant voltage circuit operation; and a voltage supply part to supplythe voltage generated by the tertiary winding of the transformer to thecontrol circuit.
 3. The converter of claim 2, wherein: the controlcircuit, if brought into a latched state by a protection circuit,provides the start-up control circuit with a latch signal whose voltageis lower than the start-up voltage and corresponds to a voltage tomaintain the latched state; and the start-up control circuit operates inresponse to the latch signal, to make the start-up circuit operate asthe constant voltage circuit.
 4. The converter of claim 1; comprising arectifying/smoothing circuit connected to an AC power source, to rectifyand smooth AC power and produce DC power, the AC power source andrectifying/smoothing circuit forming the DC power source.
 5. Theconverter of claim 1, comprising: a rectifying circuit connected to anAC power source, to full-wave-rectify an AC voltage; and a power-factorimproving converter to receive a full-wave-rectified output from therectifying circuit through a choke coil, turn on and off, rectify, andsmooth the received output with a second switching element, and providea DC output, the AC power source, rectifying circuit, and power-factorimproving circuit forming the DC power source.
 6. The converter of claim1, wherein a bias voltage or current to a control terminal of a start-upswitching element in the start-up circuit is supplied from the DC powersource.
 7. The converter of claim 5, wherein a bias voltage or currentto a control terminal of a start-up switching element in the start-upcircuit is supplied from the AC power source.
 8. The converter of claim5, wherein a bias voltage or current to a control terminal of a start-upswitching element in the start-up circuit is supplied from a front stageof the power-factor improving circuit.
 9. A converter comprising: arectifying/smoothing circuit connected to an AC power source, to rectifyand smooth an AC voltage; a first switching element connected to anoutput side of the rectifying/smoothing circuit through a primarywinding of a transformer; a control circuit to conduct ON/OFF control onthe first switching element; a rectifying/smoothing circuit to rectifyand smooth a voltage induced on a secondary winding of the transformerand provide a DC output; and a start-up circuit to start the controlcircuit, a bias voltage or current to a control terminal of a start-upswitching element in the start-up circuit being supplied from the ACpower source.
 10. A converter comprising: a rectifying circuit connectedto an AC power source, to full-wave-rectify an AC voltage; apower-factor improving circuit to receive a full-wave-rectified waveformfrom the rectifying circuit through a choke coil, turn on and off,rectify, and smooth the received output with a second switching element,and provide a DC output; a first switching element being connected to anoutput side of the power-factor improving circuit through a primarywinding of a transformer; a control circuit to conduct ON/OFF control onthe first switching element; an output rectifying circuit to rectify andsmooth a voltage induced on a secondary winding of the transformer andprovide a DC output; and a start-up circuit to start the controlcircuit, a bias voltage or current to a control terminal of a start-upswitching element in the start-up circuit being supplied from a frontstage of the power-factor improving circuit.
 11. The converter of anyone of claims 7 to 10, wherein the start-up circuit is provided with acapacitor to hold the bias voltage or current during a half period of afrequency of the AC power source.